Spring-ai Add a new test case for AnthropicAutoConfigurationIT. Considering call(null).

In addition to the existing test scenario, it would be beneficial to add a new test case that validates the behavior when the Prompt is null. This is because when the messageType is either MessageType.SYSTEM or MessageType.USER, the textContent parameter cannot be null, and an IllegalArgumentException should be thrown in such cases.

https://github.com/spring-projects/spring-ai/blob/4c83fe830211756e098e4ce72ed10a5644bb2968/spring-ai-spring-boot-autoconfigure/src/test/java/org/springframework/ai/autoconfigure/anthropic/AnthropicAutoConfigurationIT.java#L50

//This is just a sample using AssertJ
@Test
void ThrowIllegalArgumentExceptionWhenPromptIsNull() {
    contextRunner.run(context -> {
        AnthropicChatModel chatModel = context.getBean(AnthropicChatModel.class);

        assertThatThrownBy(() -> chatModel.call(null))
            .isInstanceOf(IllegalArgumentException.class)
            .hasMessageContaining("Content must not be null for SYSTEM or USER messages");
    });
}